As shown in FIGS. 1-3, a wrist ball 10 of the prior art comprises mainly a semispherical lower shell 11 of a transparent plastic material, a bracing ring 12 disposed in the inside of the top end of the lower shell 11, a rotary the inside of the top end of the lower shell 11, a rotary member 13 provided with a center shaft 131 and a groove 132 extending along the outer periphery thereof and having a hole 133. The rotary member 13 is received in the bracing ring 12 such that both ends of the center shaft 131 are fastened pivotally with the bracing ring 12. A transparent upper shell 14 of a plastic material is joined with the top end of the lower shell 11 and is provided in the top thereof with a round opening 141.
As shown in FIG. 4, a cord 20 has a head end 25, which is inserted into the hole 133 of the rotary member 13. Now referring to FIG. 5, the rotary member 13 is shown being pressed by the thumb. In the meantime, the cord 20 is wound in the groove 132 along the direction indicated by an arrow 24 such that only a small segment of the cord 20 remains outside. As shown in FIG. 6, the wrist ball 10 is held in one hand while the cord 20 is pulled forcefully with other hand along the direction indicated by an arrow 21, so as to actuate the rotary member 13 to turn in the direction indicated by an arrow 22 as shown in FIG. 7. The rotary member 13 and the bracing ring 12 are also actuated by the centrifugal force to turn in the direction indicated by an arrow 23 as shown in FIG. 8. At this moment, the wrist ball 10 is held in the palm in time, as shown in FIG. 9, such that the arm remains stationary, and that the palm is rotated on the "A" point serving as a fulcrum, so as to cause the wrist ball 10 to rotate clockwise along a circumferential track in an uninterrupted manner for testing a player's perseverance. As the wrist ball 10 is rotated forcefully, the rotational speed of the rotary member 13 accelerates to as fast as 8000 revolutions per minute. It must be noted here that wrist ball 10 buzzes at the time when the rotation of the rotary member 13 accelerates. As the palm stops turning, the rotary member 13 decelerates gradually to a complete stop.